1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery comprising a negative electrode, a non-aqueous electrolyte, and a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a binder, and a conductive agent. More particularly, the invention relates to such a non-aqueous electrolyte secondary battery employing a positive electrode containing an olivine-type lithium-containing metal phosphate as the positive electrode active material that exhibits improved discharge performance and charge-discharge cycle performance at high current discharge.
2. Description of Related Art
In recent years, non-aqueous electrolyte secondary batteries have been widely in use as a new type of high power, high energy density secondary battery. Non-aqueous electrolyte secondary batteries typically use a non-aqueous electrolyte and perform charge-discharge operations by transferring lithium ions between the positive electrode and the negative electrode.
Generally, these types of non-aqueous electrolyte secondary batteries often use positive electrode active materials such as lithium cobalt oxide LiCoO2, spinel lithium manganese oxide LiMn2O4, and lithium-containing metal composite oxides represented by the general formula LiNiaCobMncO2 (wherein a+b+c=1).
However, there have been some problems with this type of non-aqueous electrolyte secondary battery. For example, since the positive electrode active material contains scarce natural resources such as cobalt, manufacturing costs tend to be high and it is difficult to ensure a stable supply.
In recent years, the use of an olivine-type lithium-containing metal phosphate represented by the general formula LixMPO4, where M is at least one element selected from the group consisting of Co, Ni, Mn, and Fe, and 0<x<1.3, has been considered as an alternative to the above-mentioned positive electrode active materials.
The olivine-type lithium-containing metal phosphate, however, has a very high electrical resistance. The non-aqueous electrolyte secondary battery that uses the olivine-type lithium-containing metal phosphate as the positive electrode active material in its positive electrode shows a high resistance overvoltage and a low battery voltage when discharged at high current. Therefore, sufficient battery performance cannot be obtained.
In view of this problem, various proposals have been made in recent years for the batteries that use an olivine-type lithium iron phosphate, one type of the olivine-type lithium-containing metal phosphate, for the positive electrode. For example, Japanese Published Unexamined Patent Application No. 2002-110162 proposes a positive electrode active material that uses a composite material of lithium iron phosphate and a carbon material. Japanese Published Unexamined Patent Application No. 2002-110165 proposes a method of producing a positive electrode active material in which the contact area of lithium iron phosphate with a conductive agent is increased by reducing the particle size of the lithium iron phosphate.
Nevertheless, even when a composite material of lithium iron phosphate and a carbon material is used for the positive electrode active material or the particle size of lithium iron phosphate is reduced to increase the contact area of the lithium iron phosphate and a conductive agent as described above, the battery performance at high current discharge has not been improved sufficiently, and problems have remained such as the deterioration in charge-discharge cycle performance at high current discharge.
Another problem is that when the amount of the conductive agent added to the positive electrode is increased in order to reduce the electrical resistance of the positive electrode that uses the olivine-type lithium iron phosphate as the positive electrode active material, the relative proportion of the positive electrode active material in the positive electrode reduces, and consequently, the battery capacity becomes insufficient.
Furthermore, the following problems arise when lumped, or massive, carbon (“kaijo tanso”) is used as the conductive agent in the above-described positive electrode; the positive electrode mixture slurry used in preparing the positive electrode shows poor coatability, and the positive electrode shows a large volumetric change after the initial charge-discharge process. As a consequence, the conductive path originating from the lumped carbon is disconnected, and the electrical resistance in the positive electrode cannot be reduced sufficiently. In addition, the charge-discharge cycle performance deteriorates due to the heat produced during high current discharge. On the other hand, when carbon fiber is used as the conductive agent in the positive electrode, it is difficult to reduce the internal resistance of the battery below a certain level, and the battery performance at high current discharge cannot improve sufficiently.